In certain prior art auto-focusing microscopes, a light source is used to transmit light to a spot on the object to be viewed (target). The light is reflected from the surface of the target to a photodetector which senses the intensity of the reflected light. An electronic focus signal, which is proportional to the intensity of the reflected light, is generated by the photodetector. This electronic focus signal reaches a maximum (or minimum) when the target is in focus. The electronic focus signal is used to control the auto-focus operation.
In other auto-focus microscopes a white light camera is used to detect a maximum contrast in the camera picture to indicate when the target is in focus. Measuring the picture contrast requires a large amount of computer processing. This method is therefore slow and complicated. Additionally, this method fails if the target has no contrast (i.e., if the target is flat and has a uniform color, or is mirror-like).
Speed is one desirable characteristic of an auto-focus system. A larger period of time required to perform an auto-focus operation reduces the number of targets which can be viewed in a given time period, or, equivalently, increases the amount of time required to view a given number of targets, and thereby increases the cost associated with viewing each sample.
Accuracy and repeatability are other desirable characteristics of an auto-focus system. Certain prior art auto-focusing microscopes rely on a single point within the viewing area on the target to provide focusing information. In such microscopes, local height variations in the surface of the target can cause the focused condition to be less than optimal within the viewing area if the point at which focusing is performed is significantly higher or lower than the remainder of the viewing area. Certain other prior art auto-focusing microscopes utilize auto-focus optics which are separate from the imaging optics of the microscope. In such microscopes, any "drift" between the auto-focus optics and the imaging optics will result in decreased auto-focusing accuracy.
It is therefore desirable to have an auto-focus system which can perform auto-focus operations quickly, reliably and accurately.